1. Field of the Invention
This invention relates to detergent compositions with a selected type of mixed anionic surfactant system.
2. The Prior Art
Linear alkylbenzene sulfonate (LAS) is one of the most widely used surfactants in commerce. It finds special application for light and heavy duty liquid and powder detergents. A disadvantage of LAS is that this anionic material can interact with cationic water hardness ions, such as calcium, thereby becoming inactivated through precipitation. While this is a problem common to anionic surfactants, LAS is especially sensitive to water hardness ions.
Although not wishing to be bound by any theory, the literature indicates that the aforementioned interaction can best be understood by considering the micellar structure of LAS. Repulsive forces between negative charges on the sulfonate group lead to a higher critical micelle concentration (CMC) than, for instance, with a nonionic surfactant. CMC is the surfactant concentration at which micellar formation begins. Stated otherwise, the negative charge of LAS inhibits micellar formation and shifts the equilibrium towards the monomer. A relatively high monomer concentration in solution results thereby; this is significant because precipitation between calcium ion and LAS occurs only with the monomer.
Reduction of free LAS concentration (and thereby diminishing LAS sensitivity to calcium) may be accomplished by adding a co-active. A mixed micelle forms between the LAS and co-active at a lower CMC. The co-active helps shield negative charges of LAS from each other in the micelle. Micellar formation is, accordingly, more energetically favorable and lowers free monomer concentration.
Mixed micelle formation may help prevent LAS precipitation in another way. Anionic micelles can complex ions by counterion interaction. An increase in micellar concentration leads to a reduction in free calcium ions with a concomitant decrease in LAS precipitation.
Certain ethoxylated fatty alcohols have been found to be useful co-actives for protecting LAS against precipitation. It is believed that the ethylene oxide chain helps dissipate repulsive forces between negatively charged sulfonate groups of LAS. Powders with the optimum nonionic co-active are, however, generally difficult to process and have poor physical properties.
Alcohol ethoxysulfates have been suggested as co-actives in combination with LAS. Illustrative of such disclosures are U.S. Pat. No. 3,758,419, U.S. Pat. No. 3,892,680, U.S. Pat. No. 4,049,586 and U.S. Pat. No. 4,487,710. These patents do not, however, provide instruction as to the optimum alcohol ethoxysulfate relative to an LAS surfactant system.
Accordingly, it is an object of the present invention to provide a detergent composition capable of optimized cleaning and readily processible.
It is a further object of this invention to provide a co-active which will enhance the effectiveness of LAS.